In the polymerization of olefinic monomers having a terminal CH.sub.2 .dbd.C&lt; grouping such as vinyl chloride with free radical catalysts, particularly those containing oxygen, it is generally desirable to stop the reaction after the polymerization has reached a predetermined state in order to obtain stable, uniform polymers. Sometimes it is also necessary to stop or slow down reactions that are proceeding at a faster rate than is desirable. Also, in many polymerization systems, it is often desirable to stop the polymerization reaction short of complete conversion so that an effective agent of some kind must be employed to stop the reaction quickly without adversely affecting the polymerization product. The problem is particularly acute when employing the more active catalysts that give extremely fast polymerization rates and with, for example vinyl chloride polymers that are subject to thermal degradation.
Polyvinyl chloride resins are also subjected to high temperature environments after the polymer is made. It is normal for the polymer to be subjected to steam stripping, drying, and processing operations all of which subject the polymer to some thermal degradation and are normally performed at a higher temperature than the polymerization temperature. Steam stripping can reach temperatures of about 140.degree. C. and subject the polymer to degradation.
Polyvinyl chloride may be produced by the suspension, micro-suspension, or emulsion process whereby vinyl chloride monomer droplets are suspended and polymerized in an aqueous medium. Another polymerization method is to use a modified suspension process as is taught by U.S. Pat. No. 4,435,524 (incorporated herein by reference) wherein the aqueous medium is in a thickened state. This thickened aqueous suspension process produces spherical shaped resin particles which have obvious advantages. The resins spherical shape can be determined by comparing the ratio of the minor axis with the major axis using microscopic techniques well known in the art. A perfect circle would have a ratio of 1.0. The resins are considered spherical if they have a ratio greater than 0.9. Each of these processes present unique problems associated with shortstopping the polymerization reaction. Shortstop agents are normally employed in the typical suspension polymerization process to counteract this degradation and to control the resin properties. In the modified suspension process (thick aqueous medium), some shortstops that are effective for the normal suspension process will not work in the thickened medium. Apparently, they will not diffuse through the thickened medium and into the polymer. Also, resins which have low porosity are difficult to shortstop because of the difficulty in the shortstop agent diffusing into the low porosity resin.
A still further polymerization method is the bulk or mass process. The mass process normally has just the monomer(s) and initiator. Water is not present. The process depends on a two-stage polymerization. In the first stage, which is operated with strong agitation, the oil-soluble, free radical catalyst and the monomer are raised to the reaction temperature. Almost immediately, the solution becomes opalescent from formation of polymer chains, and as polymerization increases, these chains agglomerate and form small particles in the range of 0.1 micron. After about 10% conversion the liquid is transferred to a second vessel. This second vessel is horizontal and has a ribbon blender type of agitation that operates much more slowly; and in this vessel the particles are allowed to grow. The reaction medium goes through a gell state at about 30% conversion and then a wet powder state and finally a dry powder at about 50% conversion. The resin is stripped and dried in the polymerizer. The mass process leads to a resin which is free of an encapsulating skin which is also known as a pericellular membrane. Suspension process resins have a pericellular membrane, but mass process resins do not. Shortstop agents are not normally used in the mass process.
In the manufacture of homopolymers and copolymers of olefinic monomers, and particularly from vinyl halides and vinylidene halides, one of the most important aspects is reaction control during the entire course of the polymerization. This is related, directly or indirectly, to such things as demands on heat exchange equipment, reactions with such a fast rate that they become uncontrolled, formation of off-specification products, and the like. Accordingly, some means to terminate permanently or temporarily a polymerization, such as this, at any given time would be most desirable. When a polymerization plant experiences a loss of agitation or cooling ability, usually through power failure, these exothermic reactions can build up heat and pressure to the point of endangering workers and equipment. These out of control reactions must be stopped very quickly by use of what is known in the art as emergency shortstops. To be effective as an emergency shortstop, the shortstop must diffuse very quickly through the reaction medium and into the polymer.
Heretofore, various shortshopping agents have been employed to terminate the polymerization of monoolefinically unsaturated monomers. However, many of these shortstopping agents require large amounts to be effective or are toxic. Others operate by destroying the catalyst such as those disclosed in U.S. Pat. No. 3,637,632. Further, many of these shortstopping agents have been found to be ineffective in completely halting the polymerization or have been found to be effective only in such concentrations as result in discoloration of the polymer product. Free radical initiators used in the polymerization of polyvinyl chloride increase the rate of polymerization. However, these same initiators contribute to the degradation of the polymer during the stipping and drying operations that occur after polymerization.
It is a desirable goal to have an effective, non-toxic shortstop agent for shortstopping polymerization of olefinic monomers. It is desirable to have an effective non-toxic emergency shortstop. It would also be desirable to be able to shortstop and stabilize polyvinyl chloride made by the mass process and by the thickened aqueous suspension process.